1. l Field of the Invention
This invention relates generally to the cathodic protection of underground metallic structures and relates particularly to an apparatus for supporting one or more platinized anodes within a bore hole having a carbonaceous backfill which acts as a very low resistance to ground and thereby carries an impressed current from the anode into the surrounding soil.
2. Description of the Prior Art
Heretofore, it has been known that underground metallic structures, particularly structures made of ferrous metals, have corroded because such structures normally included both cathodic and anodic areas in which electric currents flowed to the metallic structure in the cathodic areas and flowed away from the structure in the anodic areas. It has been known that by providing a deep anode bed in the area of the metallic structure and causing an electric current to flow from the anode to the metallic structure, either from an impressed DC current applied to the anode, or by galvanic action, would cause the underground metallic structure to become substantially entirely cathodic and thereby reduce or prevent corrosion of such structure. Most of the anodes used in this type of protection system in the past have been sacrificial anodes in which the anodes deteriorated and after a few years it was necessary to replace the anodes. In my prior U.S. Pat. No. 3,725,669 a cathodic protection system was provided which substantially extended the life of the anode; however, the anodes still had to be changed periodically.
Some efforts have been made to provide substantially inert anodes which will not deteriorate by providing a substrate of titanium, tantalum or niobium (columbium) on which a relatively thin film or coating of a noble metal such as platinum or the like has been applied either by electrodeposition or by a cladding process. These inert anodes function for extended periods of time in the presence of a carbonaceous backfill or other electrolyte in a bore hole if the impressed current is maintained at a current discharge from the anode of approximately 20 amperes per square foot (2.15 amperes per square decimeter) or less so that the discharge remains as an electronic discharge instead of an electrolytic discharge. Ordinarily, the impressed current being discharged from the anode into the carbonaceous backfill is an electronic discharge and, therefore, little or no gas is generated. However, what little gas is generated normally is oxygen and perhaps some hydrogen and, when a chlorine ion is present, a small amount of chlorine gas likewise may be generated. Such gases normally occur at the periphery of the carbonaceous backfill and migrate to the area of the anodes where they tend to act as insulators and raise the resistivity to the passage of current. Therefore, it is important that means be provided for exhausting the gases from the area of the anodes. In offshore installations in which the electrolyte is sea water, the gases are free to pass through the water or be absorbed thereby. However, in deep well land installations, such gases may be trapped exteriorly of the anode carrier within the carbonaceous backfill material.
Some other examples of prior art structures are the U.S. Pat. Nos. to Dorr 3,458,643; Mehandjiev 3,647,672; Caldwell et al 3,769,521; as well as "Texas Instruments Clad Metal Anode Products", Texas Instruments Inc., Clad Metal Anode Products, Attleboro, Mass. 02703, Publication No. 186; and by an article entitled "The Use of Platinum Anodes on Land-Based Installations" published 1975 at a Symposium by Marston Excelsior Ltd., Wolverhampton, England.